Method and apparatus for manufacturing microfine metallic powder



Nov. 28, 1967 WATARU lsHiBAsl-n 3,355,279

METHOD AND APPARATUS FOR MANUFACTURING MICROFINE METALLIC POWDER Filed May 18, 1965 INVENTQR. Wotaru Ishlboshl ATTORNEY United States Patent Office 3,355,279 Patented Nov. 28, 1'967 ABSTRACT OF THE DISCLOSURE Method and apparatus for the disintegration of metals in liquid media by spark discharges induced by pulse voltage. Coarse metallic grains, in suspension, act as migratory electrodes to give rise to spark discharges which disintegrate said grains successively into finer fragments. The liquid media include water, hydrocarbons, liquid nttrogen, liquefied argon, and other liquid gases inert to the metals.

Background of the invention The present invention relates to a novel method for manufacturing microflne particles of general metals and alloys. It is conventional to obtain microfine metal particles whose average diameter is of the order of less than 0.1 millimeter by certain mechanical means, such as grinding a metal directly, crushing it by means of high impact, or spattering a molten mass of the metal. An alternative method that is also known is concerned with an electrolysis of the metal wherein the desired metal is electrolyzed and the resulting metal deposit on the electrode of an electrolytic cell is purified in a suitable manner. However, the metal or alloy powder prepared by such mechanical means has an intrinsic disadvantage in that not only would the product be substantially oxidized under the influence of the heat evolved but also the decrease in yield of the product owing to its flying away would be unavoidable and substantial. On the other hand, the electrical process involves a high production cost and the resulting powder itself is accordingly costly. In accordance with the present invention, the above-mentioned disadvantages of the conventional processes are completely eliminated. Furthermore, the metal or alloy powder (hereinafter referred to sometimes simply as metal powder or metal particles) obtainable according to the present invention is uniform in size and substantially spherical in shape.

Summary of the invention The present invention comprises dispersing rough grains of an electroconductive metal or alloy, for instance, in water or insulating liquid and, while the suspension is vigorously agitated, a pulsating current is injected between a pair of electrodes immersed in said dispersion of such metal or alloy pellets or grains (hereinafter referred to sometimes simply as metal grains or material grains) so that the successive series of spark discharges may occur among and between the free-flowing material lgrains which are adjacently located in random and ever-shifting patterns. As the spark discharges take place, the resulting energy of electric discharge scrapes chips off the material grains. At the same time, as the water or insulating liquid is ionized or decomposed, the forces of expansion of the gas evolved upon said ionization or gasification and the resulting forces of buoyancy further agitate the suspension. This increased agitation helps a successive series of spark discharges to take place more vigorously so that a great quantity of microfine powder may continuously be produced from the rough electroconductive metal pellets or grains.

A primary object of the present invention is to provide a simple means whereby a substantially spherical microfine powder may be produced from comparatively rough metal or alloy materials. Another object of the invention is to provide an economically feasible method whereby a substantially spherical and unoxidized microfine metal or alloy powder may be produced in one operation and in high yields. Other objects and advantages of the present invention will hereinafter be described in detail, reference being had to the accompanying drawings in which:

Brief description of the drawing FIG. l is a schematic diagram showing the simplest of the equipment to be employed in carrying the present invention into practice, and

FIG. 2 is a schematic diagram illustrating another ex ample of the equipment to be used according to the in vention.

Description of the preferred embodiments Referring, now, to FIG. l, a reaction tank 1 consists Of an outer tank 2 and a fluid tank 3 which is an upright hollow cylinder located inside the outer tank 2. The fluid tank 3 is open at both its upper and bottom ends, the botttom end being constricted to form a cranked pipe 4 extending centrally through the bottom of said outer tank 2. In the fluid tank 3 is disposed a pair of electrodes 5 and 6 which are iron bars. A bottom plate 7 having a multiplicity of small orifices is fitted across the bottom of said fluid tank 3. The upper ends of the electrodes 5 and 6 are connected to a pulsating-current generator 9 located outside the reaction tank 1 through conductors 8 and 8', respectively. The pulsating-current generator 9 is capable 0f generating a pulsating current of, e.g. about 10,000 volts and about 20-100 kilocycles per second.

Located on one side of the outer tank 2 and at a short ldistance above its bottom is a side pipe 10 which connects the fluid tank 3 to the outer tank 2. The side pipe 10 is connected to the cranked pipe 4 of the fluid tank 3 through a pump 11. Indicated by numeral 12 is a microfine-powder discharge outlet located in the bottom of the outer tank 2. In operation, the fluid tank 3 is charged with crude -material grains (A) of an electroconductive metal or alloy of the order of about 2 to 5 millimeters in diameter, and the outer tank 2 is charged with an insulating liquid (B) as shown in FIG. l. In this particular mode of embodiment of the present invention, the insulating liquid is heavy oil. Now, as the pump 11 is driven, the heavy oil B is vforced into the fluid tank 3 through the cranked pipe 4 and, then, through the orifices provided in the ybottom plate 7.

Then, as the oil flows upwards, the crude material grains are constantly agitated. If, in this state of turmoil, a pulsating current of, eg., 60 kc./s. is injected between the electrodes 5 and 6, a series of spark discharges occurs between and among the rough grains, whereby the grains are vigorously spattered while the heavy oil bubbles copiously. The force of expansion arising from the formation of bubbles and the resulting buoyancy tend to further agitate the grains and, accordingly, the spark discharges occur more vigorously. As a result, the rough metal grains are pulverized, so to speak, into finer particles which are spherical in shape as hereinbefore described. Although the mechanism underlying the above phenomena remains yet to be fully elucidated, it may presumably be explained as follows. Thus, a high potential gradient is created among the vigorously movin-g grains due to the extremely thin insulating oil films existing between and among the metal grains which, as mentioned above, are dispersed in close proximity with one another in ever-shifting patterns. The tremendous energy accumulated in this manner ultimately destroys the insulating films, whereby spark discharges take place among the metal grains. The energy which is produced by the spark discharges, in turn, decoriiposes the oily iilnis into their gaseous components and, at the same time` the electric impact scrapes away the surfaces ot the individual grains involved.

The heavy oil overflowing the fluid tank 3 is received in the outer tank 2 and recycled into the tluid tank 3 through the side pipe and the pump 11. It is to be noticed that the heavy oil overflowing the liuid tank 3 contains a great amount of microfine particles scraped oflc the crude material grains, which fall under their own weight to the bottom of the tank where a deposit formed. The deposit is taken out from the tank through said outlet 12. The deposited powder may be separated from the heavy oil in the conventional manner to obtain purified metal particles. The above arrangement, as aforesaid, is only one mode of embodiment of the principles oi this invention. In another mode of embodiment. the present invention employs water in place ot` an insulating liquid such as heavy oil. The advantage of using water obviously lies in its low cost. it is to be understood, however that there arises a marked difference in results between these alternatives. For when water is used as the reaction medium, the resulting microtine metal or alloy particles are oxidized. Thus, in another aspect. the present invention relates to a process for manufacturing oxidized microline metal or alloy powders, comprising dispersing crude material grains of an electroconductive metal or alloy in a water-containing vessel in which a pair of electrodes is disposed and injecting a pulsating current between said electrodes. In this arrangement, the suspension of said rough grains is agitated under the intiuence of the resulting electric impact and, at the same time, a series of spark discharges takes place among said rough grains, whereupon the resulting energy o electric discharge causes the individual grains to be scraped-off step by step into substantially innumerable microtine metal or alloy particles and, substantially at the same time, causes said metal or alloy particles to be oxidized by the oxygen liberated upon ionization of the water. It will be apparent that the second-mentioned mode of embodiment of the present invention is preferred to the first-mentioned when the tinal microne powder may not necessarily be unoxidized.

FIG. 2 is another example of the equipment adapted to carry the present invention into practice. Although the following description relates to the mode of embodiment of the present invention in which water is used as the reaction medium, it may be an insulating liquid, eg., heavy oil, just as described with reference to FIG. l.

Referring to FIG. 2, where like parts are indicated by the same numerals as in FIG. 1 unless otherwise specilied, a reaction tank 1 consists of an inner tank 3 and an outer tank 2, and a pair of electrodes 5 and 6 is disposed in said inner tank 3. The upper ends of the electrodes 5 and 6 are connected to a pulsating-current generator located outside the reaction tank through conductors 8 and 8', respectively. The capacity of the pulsating-current generator 9 is the same as that for the equipment of FIG. 1. In order that both the agitation of material grains and the collection of final microiine particles may be carried out etectively, the bottom of the outer tank 2 is inclined by, e.g., 45 degrees. The lower part of the inner tank 3 is disposed below the inclined bottom of the outer tank 2. As in FIG. l, a bottom plate 7 having a multiplicity of small orifices is fitted horizontally within and near the bottom of the inner tank 3, and the lowermost portion of the outer tank 2 is connected to the bottom of the inner tank 3 through a pipe 10, a storage tank 1S, a pipe i4, a pump 11, and a pipe 4 in that order. Indicated by numeral 13 is an inlet-outlet, through which material metal or alloy grains are charged into the inner tank 3 and through which the gases may escape. In this arrangement, water is admitted into the reaction tank and a suitable amount of crude grains into the inner tank 3. The crude grains may be obtained, for

example. 'ny cutting a rod or wire of about 2 mm. diameter :nto 5 mm. lengths. Prior to the starting ofthe equipment, these pellets remain piled on the base plate 7 in the inner tank 3. However. when a pulsating current of, eg., 6() kilocvcies per second is injected between the electrodes S .ind 6. the resulting electric impact boils the n ater, w hereby the pellets are spattered around and, in the process. those grains which happen to come into Contact with each other in random patterns take part in a series of spark discharges in the water. As these spark dis- ;iiarges take piace. part of the water is ionized and gases are `vigorously evolved. The impact of these spark discharges in the water and the evolution of gases further .igitate the water .ind its .i result. the crude pellets are further vigorously involved in the spark discharge. ln this manner. the two phenomena of agitation and spark discharge take place substantially concurrently. Now, when the pump 11 is driven to force water into the inner tank 3 through the orifices iii the bottom plate, the water containing the crude pellets is further agitated. The water and gases. both of which are lower in specific gravity than the metal pellets, and the particles formed upon said spark discharges overflow the tank 3 into the outer tank 2 L1nd the gases further escape through the outlet 13. The water in the outer tank 2 falls into the storage tank 15 through the pipe 10, whence it llows to the pump 1l, which recycles it into the inner tank 3 through the pipe 4. This operation is repeated time and again without intermission. The water which enters the storage tank 15 as described above contains a great number of microfine metal particles, which precipitate in the bottom of the tank at C. These particles are then taken out, dehydrated. and purified in the conventional manner.

In still another mode of embodiment of the present invention, the reaction medium is selected from the group consisting of liquefied inert gases such as liquid nitrogen and liquid argon. It is to be understood, however, that when such a liquefied gas is employed, the equipment needs to be properly insulated. This insulation may be easily provided in any conventional manner, and the process is advantageous in that the product is automatically puried as the liquefied gas is evaporated as soon as it is taken out of the equipment. In this connection, it is to be noted that unlike the case in which a hydrocarbon is used as the reaction medium, no carbon is produced and. therefore, there is no need to remove any such byproduct from the system. This feature compensates fully for the additional cost involved in the provision of said insulation.

As hereinbefore described in detail, the microiine metal powder obtainable according to the present invention is of the order of about 0.05 to 3 microns in diameter, substantially irrespective of the types of material metal, eg., iron, aluminum, nickel, lead, and the like, and, under microscope. each particle appears as a sphere resembling a droplet of water. It is to be noted, however, that there are obtained markedly ditierent results according to whether the reaction medium is water or an insulating liquid. Thus, whereas the use of an insulating liquid yields unoxidized metal or alloy particles, the use of water gives rise to microtine particles which have been oxidized almost to the core.

A notable feature of the present invention is that the product is substantially uniform in size. Secondly, but no less important, the present invention makes it possible to manufacture microfine metal or alloy particles in any desired narrow size range by selecting the proper reacting conditions. Roughly speaking, this result may be attained by allowing the reaction to continue either for a short time or for a long time. To attain the same result as above, it is also possible to vary other conditions, such as the voltage and frequency of the pulsating current to be injected, for instance.

When water is used as the reaction medium, the power required to obtain t kg. of oxide iron, l kg. of lead peroxide, and l kg. of aluminum oxide are 4.1 kw./hr., and 5.5 kw./hr., respectively. In these cases, the volumetric amounts of the final powders range from about 60 t0 about 70 cubic centimeters. The water temperature, or the reaction temperature, remains about 40-60 C. throughout the entire process. This is another valuable advantage of this invention. While, as aforesaid, the precise mechanism involved in the process of this invention remains yet to be fully elucidated, and examination of the surface of the semi-iinished particle reveals that it has apparently been chipped or scraped ofi. The electrodes used in this invention should be of the same material as the metal grains, for the electrodes are also scraped off, although to much less extent, in the course of reaction. However, the scraping of the electrodes is so minimal that they need not be replaced with new ones over long periods of time. The distance between the positive and negative electrodes depends upon the peak voltage of the pulsating current to be used, but generally speaking, it should be such that no arc discharge occurs, e.g., 30 mm.-150 mm. A pulsating current is used, instead of an alternating or a direct current, to cause sparking between metal grains and also between the water or insulating liquid and the individual grains and, accordingly, to facilitate the occurrence of spark discharges in the liquid medium. In case water is used as the reaction medium, it was feared earlier that the hydrogen and oxygen formed upon electrolysis of the water might be ignited by the sparks and explode, that the water might boil due to the electrical resistance, or that the impact of electric discharge might result in a tremendous building-up of pressure in the water and, consequently, a major disaster. These fears have been found entirely groundless when the equipments illustrated and described hereinbefore have actually been operated.

It will be apparent from the above description that the present invention provides a simple means whereby a line metal or alloy powder of uniform size may be produced in one operation and at nominal cost.

What I claim as my invention:

1. A method of manufacturing microne metallic powder, which comprises: charging a reaction chamber containing a pair of electrodes and water with coarse grains of a metallic material; impressing a pulsating voltage between said electrodes .of such magnitude that spark discharges occur among said grains devoid of arcing between said electrodes, the energy of said spark discharges comminuting said coarse grains and producing iner metallic particles.

2. A method substantially as claimed in claim 1, wherein an insulating liquid is employed in place of water.

3. A method described in claim 2, wherein said insulating liquid is a member selected from the group consisting of petroleum type liquid hydrocarbons and the liquid hydrocarbons of the animal and of the vegetable oil type.

4. A method described in claim 2, wherein said insulating liquid is selected from the group consisting of liquid nitrogen, liqueed argon, and other liquelied inert gases.

5. A method as described in claim 1, wherein the magnitude of said pulsating voltage is about 3,000 to 100,000 volts and its pulse repetition rate is about 20 to 100 kilocycles per seconds.

6. A method substantially as described in claim 1, wherein the gases evolved upon the ionization of water are utilized to further agitate said grains so that the spark discharges may occur more vigorously.

7. A method substantially as described in claim 3, wherein the gases evolved upon decomposition of said insulating liquid are utilized to agitate grains so that spark discharges may occur more vigorously.

8. A method substantially as described in claim 1, wherein said water is circulated by force to assist in the agitation of said metal or alloy grains.

9. A method substantially as described in claim 2, wherein said insulating liquid is circulated by force to assist in the agitation of said grains.

1t). A method substantially as described in claim 1, wherein said water is repeatedly recycled.

11. A method substantially as claimed in claim 2, wherein said insulating liquid is repeatedly recycled.

12. An apparatus for use in the production of microline metallic powder, comprising: a reaction tank including an inner chamber and an outer chamber; a pair of electrodes disposed in said inner chamber and at least partially immersed in water containing coarse grains of a metallic material; and means for supplying a pulsating voltage to said electrodes of such magnitude that spark discharges occur among said grains only devoid of arcing between said electrodes and the grains, the energy of said discharges comminuting said grains to yield microne particles.

13. An apparatus substantially as described in claim 12, wherein the bottom of said outer chamber is inclined to assist in the collection of said microline particles.

14. An apparatus substantially as claimed in claim 12, wherein a chute or hopper adapted to feed said grains to said inner chamber, a storage tank for said water and microfine particles, and pumping means adapted to cause the water to be circulated and recycled are additionally provided.

15. An apparatus substantially as described in claim 12, wherein said electrodes are made of the same material as the said grains.

16. An apparatus as claimed in claim 12 wherein an insulating liquid is employed instead of Water.

17. An apparatus substantially as described in claim 16, wherein a hopper or inlet adapted to supply said reaction tank with said grains and said insulating liquid and a pumping means adapted to circulate said insulating liquid are additionally provided.

18. An apparatus described in claim 16, wherein said electrodes are made of the same material as said grains.

19. An apparatus substantially described in claim 16, wherein all the elements to be exposed to the insulating liquid are insulated.

References Cited UNITED STATES PATENTS 1,787,659 1/ 1931 Bischoff 252-313 2,098,710 11/ 1937 Phillipi et al 252--313 2,162,823 6/ 1939 Phillipi et al. 252-359 DAVID L. RECK, Primary Examiner. W. W. STALLARD, Assistant Examiner. 

1. A METHOD OF MANUFACTURING MICROFINE METALLIC POWDER, WHICH COMPRISES: CHARGING A REACTION CHAMBER CONTAINING A PAIR OF ELECTRODES AND WATER WITH COARSE GRAINS OF A METALLIC MATERIAL; IMPRESSING A PULSATING VOLTAGE BETWEEN SAID ELECTRODES OF SUCH MAGNITUDE THAT SPARK DISCHARGES OCCUR AMONG SAID GRAINS DEVOID OF ARCING BETWEEN SAID ELECTRODES, THE ENERGY OF SAID SPARK DISCHARGES COMMUNTING SAID COARSE GRAINS AND PRODUCING FINER METALLIC PARTICLES. 